1. Field of the Invention
The present invention relates to an automatic distortion-compensating amplifier system.
2. Description of the Related Art
FIG. 1 shows a typical prior art circuit configuration for automatically compensating a distortion generated in a main output signal So which is output from a power amplifier. In a distortion compensation circuit 2 a main input signal Si input thereto is added with a compensation signal which is generated in compensation signal generator 2-1. The compensation signal is generated equivalent in level but opposite in phase to the distortion component generated in power amplifier 1, that is, so as to cancel the distortion in advance. A monitor signal Sm picked up from main output signal So via a directional coupler 7 and a divider 9 is detected by a distortion detecting circuit 30 so as to output via filters a lower- and higher-band distortion level indicating signals V.sub.L and V.sub.H. Phase and amplitude of the compensation signals is controlled in the distortion compensation circuit by control signals output from a control circuit 40 so as to keep levels of the lower- and higher-band distortion level indicating signals, V.sub.L and V.sub.H, minimum. A division of the main input signal Si divided by directional coupler 6 and another monitor signal Sm divided by divider 9 are input to an AGC (automatic gain control) circuit 5 which controls the gain of power amplifier 1 to be constant.
FIG. 2 shows a circuit configuration of prior art distortion detecting circuit 30. Monitor signal Sm from divider 9 is input via a fixed attenuator 351 to a bandpass filter 352 which allows signal band as well as distortion component spectrums existing on both higher- and lower-outsides of the signal band, to pass therethrough, so as to reject unnecessary noise spectrums. An output signal from bandpass filter 352 is input to a mixer 353, where the input signal is converted to an IF (internal frequency) signal by being mixed with a local frequency signal from a local oscillator 354. IF signal output from mixer 353 is input to a band-elimination filter 356, where only the signal band is rejected, but both the higher-band and lower-band distortion spectrums near to the signal band are allowed to pass therethrough. Signal output from band-elimination filter 356 is equally divided by a divider 357, output signals of which are input to a lower-band distortion detecting circuit 358 as well as to a higher-band signal detecting circuit 359. Lower-band distortion detecting circuit 358 includes a bandpass filter 361 for allowing the lower-band spectrum of the distortion to pass; an amplifier 362 for amplifying the output of bandpass filter 361; a detector 363 for detecting mean amplitude of the amplified distortion component; and an amplifier 364 for amplifying the detected signal, where the output of amplifier 364 is a lower-band distortion level indicating signal V.sub.L. In a similar way, higher-band distortion detecting circuit 359 includes a bandpass filter 365 for allowing the higher-band spectrum of the distortion to pass; an amplifier 366 for amplifying the output of bandpass filter 365; a detector 367 for detecting mean amplitude of the amplified distortion component; and an amplifier 368 for amplifying the detected signal, where the output of amplifier 368 is a higher-band distortion level indicating signal V.sub.H. The outputs, V.sub.L and V.sub.H, from distortion detecting circuit 30 are input to control circuit 40, which controls phase and level of the distortion compensation signal in distortion compensating circuit 2 so that the levels of thus detected distortion components, V.sub.L and V.sub.H, become minimum.
Problems in the above described prior art configuration include the following. The distortion spectrums generated in the output signal appear generally in the immediate vicinity of the signal band, due to the narrow frequency spacing of the multi-channel signals. Therefore, band-elimination filter 356 for rejecting the signal band is required to have a sharp cut-off characteristic, and therefore is expensive. Band-elimination filters having less sharp cut-off characteristics can not adequately suppress the signal band. The remaining signal component prevents an accurate detection of the distortion component, accordingly, prevents distortion compensation circuit 2 from sufficiently accurately compensating the distortion.
Moreover, when the main input signal is small, such as when the number of the multi-channel carriers is small, the detection of distortion becomes difficult, resulting in unnecessary optimizing operation of control circuit 40 to seek for the minimum distortion conditions.